An Introduction to Socket Programming

By Reg Quinton

These course notes are directed at Unix application programmers who want to develop client/server applications in the TCP/IP domain (with some hints for those who want to write UDP/IP applications). Since the Berkeley socket interface has become something of a standard these notes will apply to programmers on other platforms.

Fundamental concepts are covered including network addressing, well known services, sockets and ports. Sample applications are examined with a view to developing similar applications that serve other contexts. Our goals are

• to develop a function, tcpopen(server,service), to connect to service.
• to develop a server that we can connect to.

This course requires an understanding of the C programming language and an appreciation of the programming environment (ie. compilers, loaders, libraries, Makefiles and the RCS revision control system).

Netstat Observations:
Inter Process Communication (or IPC) is between host.port pairs (or host.service if you like). A process pair uses the connection -- there are client and server applications on each end of the IPC connection.

Note the two protocols on IP -- TCP (Transmission Control Protocol) and UDP (User Datagram Prototocol). There's a third protocl ICMP (Internet Control Message Protocol) which we'll not look at -- it's what makes IP work in the first place!

TCP services are connection orientated (like a stream, a pipe or a tty like connection) while UDP services are connectionless (more like telegrams or letters).

We recognize many of the services -- SMTP (Simple Mail Transfer Protocol as used for E-mail), NNTP (Network News Transfer Protocol service as used by Usenet News), NTP (Network Time Protocol as used by xntpd(8)), and SYSLOG is the BSD service implemented by syslogd(1M).

The netstat(1M) display shows many TCP services as ESTABLISHED (there is a connection between client.port and server.port) and others in a LISTEN state (a server application is listening at a port for client connections). You'll often see connections in a CLOSE_WAITE state -- they're waiting for the socket to be torn down.

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